The present invention relates generally to hydraulic pumps.
Generally, a fluid powered system, e.g., steering system or transmission system, which is of a hydraulic design uses hydraulic pressure and flow to provide the required fluid power to the system. However, the hydraulic fluid must be pumped and regulated. The hydraulic pump creates the hydraulic force and typically a flow control valve regulates the flow. A conventional vane-type pump comprises a cam (pump) ring having a substantially elliptical cam surface, a rotor which is adapted to rotate within the cam ring and a plurality of vanes adapted to move back and forth within radial slits formed in the rotor. The cam ring is stationary and the outer edges of the vanes touch the inside of the surface of the cam ring. Because of the substantially elliptical shape of the cam ring, the vanes slide in and out of their slots and maintain contact with the inside surface of the cam ring as the rotor turns therein. The volume of each pumping cavity constantly changes due to the elliptically shaped cam ring. Volume increases as the vanes move through the rising portion of the cam ring, drawing fluid through an intake port. When the vanes move into the xe2x80x9cfallingxe2x80x9d portion of the ring contour, volume decreases. Decreased volume increases pressure, forcing fluid out through the discharge port. An intake portion of the hydraulic pump receives low-pressure hydraulic fluid from a pump reservoir. Discharged fluid, under high pressure, flows to a desired system location (e.g., a steering gear to provide power assist).
In fixed displacement pumps, at low engine speeds, the operating system can handle the volume of hydraulic fluid provided by the pump. Flow dramatically increases at higher speeds because the pump draws and discharges a greater volume of fluid. However at high speed operating conditions, the volume of the discharged fluid exceeds the demand of the system but due to the design of the pump, the pump is required to direct all the fluid from the pump and throughout the system. These conditions raise operating temperatures and reduce pump durability. In addition, the torque necessary to drive the pump increases at higher system back pressures which corresponds to additional horsepower (energy) being required to effectively overcome the system back pressure and distribute the fluid throughout the system.
Another pump conventionally used is a variable displacement pump. A variable displacement pump provides a reduction in flow as a function of operating conditions and therefore requires more costly shaft support solutions. Additionally, since variable displacement pumps are typically single stroke, the pumps require a larger package size to provide the same pumping capacity. Variable displacement pump valving also make these pumps less efficient in the full displacement operating condition.
There is a perceived need for a fixed displacement hydraulic pump, preferably a vane-type pump, for use in a vehicle operating system, wherein the pump has improved energy efficiency while at the same time provides adequate hydraulic power.
This invention offers advantages and alternatives over the prior art by providing a dual port hydraulic fixed displacement pump which exhibits improved efficiency by limiting the volume of discharged fluid which is subjected to the line pressure of a hydraulic system through mechanical valve control. In an exemplary embodiment, the fixed displacement pump comprises a vane-type pump having a vane assembly which includes pumping cavities formed by a plurality of vanes. The constantly changing volume of these pumping cavities as the pump is driven causes fluid to be both drawn into the pumping cavities and forced out of the pumping cavities and through discharge ports of the pump.
According to the present invention, a pair of discharge ports are provided, namely a first discharge port and a second discharge port. The first discharge port fluidly communicates with a primary discharge passageway and discharge outlet which is connected to a primary line for distributing the fluid throughout the system. Under all operating conditions, e.g., low and high pump speed operating conditions, the fluid flowing within the first discharge port and primary discharge passageway is exposed to the working pressure of the primary line, which represents a high pressure line. The second discharge port fluidly communicates with a secondary discharge passageway which is in selective fluid communication with a low pressure line connected to a low pressure area of the pump (e.g., a reservoir) under first operating conditions and is also in selective communication with the first discharge port and the primary discharge passageway under second operating conditions. The first operating conditions comprise high speed operating conditions (e.g., pump speeds above 2500 rpm) where pump output exceeds system fluid demands and the second operating conditions comprise low speed operating conditions where system demands require full pump capacity.
A flow control valve is disposed within the pump and acts to direct the fluid flowing within the secondary discharge passageway according to either a second discharge path, wherein the fluid is directed to the low pressure line and the low pressure reservoir or sump of the system, or a third discharge path, wherein the fluid is directed to the primary discharge passage and is subjected to the high pressure line of the system. In an exemplary embodiment, the flow control valve comprises a hydromechanically controlled valve which is designed to actuate when the fluid flowing within the secondary discharge passageway reaches a predetermined flow rate. Upon actuation, all of the fluid flowing through the secondary discharge passageway is directed to the low pressure line instead of the high pressure line of the primary discharge passageway. As a result, only fluid flowing in the primary discharge passageway is exposed to the high pressure of the system line and the fluid within the secondary passageway is subjected to a much lower pressure in the low pressure line.
The pump preferably further includes a check valve which is placed between the primary and secondary discharge passageways to control backflow from the primary discharge passageway when the secondary discharge passageway is exposed to low pressure.
Consequently, the torque to drive the pump is significantly reduced and thus a considerable reduction in horsepower is achieved because all of the fluid is not exposed to the high back pressure of the primary line. In practice, the flow control valve is actuated under high pump speed operating conditions (e.g., above 2500 rpm) where the pump output significantly exceeds system demands. Under low pump speed operating conditions when system demands require full pump capacity, the flow control valve is not actuated and all of the fluid within the secondary discharge passageway is directed to the primary discharge passageway and is exposed to the high pressure line of the system so that the system demands are satisfied.
The above-described and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description, drawings, and appended claims.